Conventional evaporators are well known. Typically they utilize thermal conduction and buoyancy-driven convection to transfer thermal energy from heating elements to the liquid-vapour (or gas) interface where the liquid is converted to vapour. The heating element is immersed in the liquid phase below the interface. The efficiency of such conventional evaporators, in terms of their rate of evaporation relative to the temperature of the heating source is less than optimal: when the heating element is immersed in the liquid, there is a temperature decrease of the liquid between the position where the heating element is placed and the liquid-vapour interface where the vaporisation process takes place. The deeper the heating element is placed below the interface, the larger the temperature decrease and the less effective the conventional evaporator design is for a given heat-source temperature at producing vapour. The amount of energy required to change a liquid phase to vapour is known (for water, for example, at 25° C. it is 2305 kJ/kg).
Thermocapillary convection is generally known as a mode of fluid transport at a liquid-vapour interface in which the fluid is transported in the direction of increasing surface tension. The surface tension generally decreases as the temperature is increased. Thus, imposing a temperature gradient along an interface can generate thermocapillary convection. Although the presence of thermocapillary flow has been well documented for liquids other than water, those skilled in the art previously believed that thermocapillary convection did not exist for water (see H. K. Cammenga, D. Schreiber, G. T. Barnes, and D. S. Hunter, J. Colloid Interface Sci. 98, 585, 1984). It is also known that thermocapillary convection is present during water evaporation.
The conventional view has been that only negligible thermal energy is transported by thermocapillary convection because the surface phase is so thin. However, it has been discovered that when water evaporates while maintained at the mouth of a stainless steel funnel, up to 40% of the thermal energy required to sustain the evaporation process in steady state was transported by thermocapillary convection.
The contribution of thermocapillary convection is not optimized when evaporation takes place at the mouth of a funnel. Since evaporation is such an important industrial process and needs to take place with the highest efficiency possible, there is a need, for a given heating source temperature, for methods that maximize the amount of vapor produced per unit time and per unit liquid-gas interfacial area (i.e. the evaporation flux). There is also a need for an evaporator design that greatly improves the evaporation flux compared to the conventional design concepts of evaporators. There is a further need for an evaporator that utilizes thermocapillary convection.